Hot-air engines are known. One of the earliest examples of a hot-air engine was proposed by Sir George Cayley in 1807, which utilised a reciprocating piston compressor coupled to a reciprocating piston expansion engine. A more modern version of a known hot-air engine is a hot-air turbine engine.
Known hot-air engines suffer from numerous disadvantages, particularly when driven by relatively low temperature energy sources, including: poor mechanical efficiency; poor thermal efficiency; relatively large physical size relative to power output; and relatively high costs.